The Anti-Inflammatory Effect of Ciglitazone is Partially Mediated by Glucocorticoid Induced Leucine Zipper in Human Airway Epithelial Cells

Abstract

RATIONALE: PPARγ activation has been shown to inhibit airway inflammation but the mechanisms are not fully understood; we therefore assessed whether the anti-inflammatory protein Glucocorticoid-Induced Leucine Zipper (GILZ), which we have shown is upregulated by the PPARγ agonist ciglitazone, is involved in this process. METHODS: The expression of GILZ was assessed by immunoblotting in A549 and normal human bronchial epithelial cells (NHBE) at baseline and following ciglitazone treatment. RNA silencing was used to knock down GILZ protein levels in A549 cells in order to determine whether GILZ mediates the anti-inflammatory effects of ciglitazone. GILZ siRNA- and Control siRNA-treated A549 cells were stimulated for 20 hours with ciglitazone followed by IL-1beta for 4 hours. Total RNA was isolated and IL-8 mRNA levels were assessed by quantitative real-time PCR. RESULTS: GILZ levels were upregulated by ciglitazone in A549 and NHBE cells. Ciglitazone potently inhibited the induction of IL-8 mRNA by IL-1beta in control siRNA cells, whereas in GILZ siRNA cells this effect was partially abrogated. CONCLUSION: Here we demonstrate the induction of GILZ, an anti-inflammatory protein known to inhibit NF-κB and AP-1, by ciglitazone in human airway epithelial cells. Further we demonstrate that GILZ in part mediates the anti-inflammatory effects of agonist-bound PPARγ. These findings help elucidate a mechanism via which PPARγ agonists reduce airway inflammation, and suggest the potential of the thiazolidinediones in the treatment of asthma.